Inhibitor solution, and method of inhibiting oil well corrosion therewith



- Patent 2,885,359 Patented May 5, 1959 INHIBITOR SOLUTION, AND METHOD OF INHIB- ITING OIL WELL CORROSION THEREWITH Edward N. Frisius, La Habra Heights, Califl, assignor,

by mesne assignments, to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Application October 12,1954 Serial No. 461,910

3 'Claims. (Cl. 252-855) This invention relates to an inhibitor consisting of an aqueous solution of the ingredients arsenous oxide, sodium hydroxide, and boric acid. The invention also relates to a method of employing said solution to inhibit corrosion of ferrous metal surfaces in an oil well producing a well fluid comprising oil, brine, and carbon dioxide gas.

The problem of inhibiting corrosion of ferrous metal surfaces in oil wells, and particularly in oil wells producing not only oil and brine but also substantial amounts of carbon dioxide gas, is of extreme importance since unless corrision is efiectively inhibited the well must be shut down, and expensive repairs and replacements effected, at rather frequent intervals. One of the most effective and widely used inhibitor solutions for preventing such corrosion in oil wells is a solution of arsenous oxide in aqueous sodium hydroxide, the latter ingredient being necessary in order to dissolve the arsenous oxide which is the active inhibiting ingredient but is relatively insoluble in water. Such an inhibitor, which generally contains four pounds of arsenous oxide and from one to two pounds of sodium hydroxide per gallon of solution, is highly caustic and thus diflicult to handle, having a high concentration of hydroxyl ions and a pH on the order of 10.5 to 11.5.

Not only is the described arsenous oxide-sodium hydroxide inhibitor solution so caustic that it is difficult to handle, but the use of such a solution in many oil wells results in precipitation of an arsenous oxide paste or scum on the ferrous metal surfaces in the oil well, such as on the tubing in situations where introduction of the solution is through the annulus between the casing and tubing. This paste or scum of arsenous oxide is very undesirable for several reasons, the first :being that it is a poison and creates a hazard to maintenance personnel when the tubing is pulled, and the second being that the precipitate represents material which is completely ineffective for corrosion inhibition and is therefore wasted.

The formation of the arsenous oxide precipitate results from a reaction between the arsenous oxide-sodium hydroxide inhibitor solution and the carbon dioxide gas naturally present in the annulus through which the solution is introduced. More specifically, the carbon dioxide gas is absorbed by the water in the inhibitor solution and this forms carbonic acid, which neutralizes the sodium hydroxide and results in precipitation of arsenous oxide. The precipitation of arsenous oxide occurs after sufiicient carbonic acid has been formed to lower the pH of the inhibitor from 10.5 to 11.5 to approximately 8.5, a pH at which the solution is no longer sufficienfly basic to hold the relatively insoluble arsenous oxide in solution.

In view of the above factors characteristic of corrosion inhibitor solutions consisting of arsenous oxide dissolved in aqueous sodium hydroxide, it is an object of the present invention to provide a simple and inexpensive inhibitor solution having a relatively low pH, and which is not subject to arsenous oxide precipitation during introduction into an oil well.

A further object of the invention is to provide a method of inhibiting the corrosion of ferrous metal surfaces in an oil well producing a well fluid comprising oil, brine and carbon dioxide gas, without resulting in precipitation of arsenous oxide on ferrous metal surfaces in the well.

A further object of the invention is to provide an inhibitor solution which is relatively simple and economical to manufacture, which has a pH sufficiently low to facilitate its'handling, and which will not precipitate arsenous oxide when introduced into an oil well.

These and other objects and advantages of the invention will be more fully set forth in the following specification and claims setting forth the composition of matter of the invention, and the method of inhibiting corrosion therewith.

According to the invention, an inhibitor of acid attack on ferrous metal surfaces, particularly in oil wells, is formed by dissolving in water forty parts by weight of arsenous oxide, eight to twelve parts by weight of sodium hydroxide, and eight to twelve parts by weight of boric acid. As will be discussed in detail subsequently, the amount of water in which the arsenous oxide, sodium hydroxide and boric acid are dissolved is determined largely by commercial factors, additional water being conventionally added at the well site in order to effect thorough washing of the inhibitor to the bottom of the well. A preferred inhibitor consists of four pounds arsenous oxide, one pound sodium hydroxide and one pound boric acid per gallon of aqueous solution.

As previously indicated, solutions of arsenous oxide in aqueous sodium hydroxide are presently employed commercially as inhibitors of oil well corrosion, the conventional solution being four pounds of arsenous oxide and one to two pounds of sodium hydroxide per gallon. The additionof boric acid to such a solution produces two very beneficial results, the first of which is to lower the pH or causticity of the inhibitor so that it is substantially easier to handle and ship, and the second of which is to prevent precipitation of arsenous oxide in the well annulus. The addition of boric acid to the basic arsenous oxide solution lowers the pH by combining with excess hydroxyl ions as follows:

In the preferred embodiment of the invention the pH is lowered to a value of approximately 9.

Addition of boric acid to the arsenous oxide-sodium hydroxide solution prevents precipitation of arsenous oxide in the well annulus because of a buffering action which prevents the pH from falling from about 9 to about 8.5, the latter being the pH value at which arsenous oxide precipitation will occur. This buffering action results from the following equilibrium which is functioning in the solution:

As can be seen from the equation, as the carbon dioxide (and thus carbonicacid) neutralizes the hydroxyl ions, the reaction shifts to the right so as to produce additional hydroxyl ions and thus maintain the pH of the solution. An additional reason for the non-precipitating characteristic of the invention is that the solution containing boric acid hasa lower pH when introduced into the annulus, so that there is substantially less tendency for the carbon dioxide to dissolve and react with the hydroxyl ions than in-the case of high-pH commercial inhibitor solutions. It has been found that theabove indicated range of proportions of boric acid, sodium hydroxide and arsenous oxide is practical and will result in both of the advantages discussed above. Thus, twelve parts by weightof sodium hydroxide is the maximum which should be employed for the forty parts of arsenous oxide since an excess of sodium' hydroxide over twelve parts would necessitate' the use of an undesirable amount of boric acid in order to lower the pH of the solution to 9 as desired, v

and would increase the difiiculty and cost. of manufacture. On the other hand, the minimum of eight parts by weight of sodium hydroxide to the'forty parts of arsen: 1

' oxide and eight to twelve parts by weight of sodium hydroxide, the lower limit of eight parts by weight of boric acid is selected since this is'the'minimu'm amount necessary to lowerthe pH as desired, and the upper limit of twelve is selected since this is the maximum amount which is feasible with respect to expense and facility of manufacture. It has been found that if ten parts by weight of boric acid are left in contact with a solution containing forty 'parts'by weight arsenous oxide and ten partsby Weight sodium hydroxide, all or nearly all of the 1 boric acidwill dissolve in one to two hours with only occasional stirring andat room temperature. I 1

The water content is important relative to storage of the solution, transportation to the well site and handling in'well treatment. The minimuinamount of water is determinedby the solubility of the components and by the viscosity of the solution, solutions containingsix pounds of arsenous oxide per gallon being about asconcentrated tion containing fourpounds of arsenous oxide per gallon.

Stated in terms of parts by Weight, the minimum amount of water is sixty parts by weight and the'maximum is three. hundred fifty parts by weightfor the, solution of 1 forty parts by weight arsenousoxide, eight to twelve parts by weight sodium hydroxide and eight to twelve partsby weight boric acid.

-.'A preferred method of manufacture is to add one pound of boric acid to a commercial inhibitor containing four pounds of arsenous oxide and one pound of sodium hydroxide per gallon of aqueous solution. Alternatively,

a solution may be made by dissolving one pound of sodium hydroxide in water and then adding four pounds of arsenous oxide and dissolving through heating and stirring, after which one pound of boric acid is added and stirred until dissolved. Where the boric acid is added to a commercial solution of four pounds arsenous oxide and one pound sodium hydroxide per gallon of solution, it is unnecessary to efiect any heating since the pound of boric acid will dissolve into the arsenous oxide-sodium hydroxide solution at room temperature and in one to two hours as indicated above. Although the above description of method of manufacture relates to the preparation of the preferred commercial solution comprising four pounds of arsenous oxide, one pound of sodium hydroxide and one pound of boric acid per gallon of aqueous solution, it is to be understood that the ranges of eight to twelve parts sodium hydroxide and eight to twelve parts boric acid also apply with relation to the method of manufacture.

In carrying out the method of the invention, corrosion of ferrous metal surfaces in oil wells is mitigated by introducing the above inhibitor solution into the well, preferably by continuous pumping of small quantities of the inhibitor through the annulus between the well casing and tubing. It is to be understood, however, that the solution may also be introduced in other ways, such as by lubricating relatively large quantities of the inhibitor periodically into the annulus. Preferably, and in order to insure that the solution will reach the well bottom, a relatively large amount of Water is introduced with the inhibitor. The added water is preferably in the range of from fifteen to thirty gallons per gallon of inhibitor solution. The solution is introduced into the well bottom in I amounts such that the water produced by thewell will have an inhibitor concentration: of fromone. to one hundred parts, per million (p.p.m.), calculated as arsenous oxide. The preferred inhibitor concentration in the produced water is five parts per million calculated as arsenous oxide. I

It has been found that the inhibitor of the invention is completely stable, samples having been stored for several months without any signs of precipitation or decomposition- The pH of the preferred solution consisting of four 7 pounds arsenous oxideand .one. pound :each of sodium hydroxide andboric acid per gallon was found to be 8.8,

which is relatively non-caustic and safe and easy to handle. Furthermore, the inhibitor and the method of its introduction have'been tested in oil Wells which formerly were characterized by precipitation of arsenous oxide after introduction of the conventionalsodium hydroxide as may be employed. The'rnaximum' amountof water is selected as one pound of arsenous oxide per gallon,

. greater amounts of water being vunnecessary and economically unjustified. The preferred concentrationis a solui 8 to 12 parts by weight ofsodium hydroxide,

and arsenous oxide solutions. It was found that with the solutions and method of the invention no precipitation of arsenous'oxide or other ingredients occurred.

-While the particular com-position of matter and method herein described in detail are fully capable of, attaining the objects and'providing :the advantages hereinbefore stated, it is to be understoodthat they are merely illus trative of thepresently preferred embodiments of the invention and that no limitations areintended otherthan as defined in the appended claims.

I claim:

1. As an inhibitor for preventing corrosion of ferrous Jmetal surfaces in oilwells, asolution consisting of the following ingredients:

40 parts by weight of arsenous oxide,

8 to 12 parts by weight of boric acid, and

60 to 350 parts by weight ofwater said solution being characterized by the capacity to dissolve substantial amounts of carbon dioxide without precipitation of arsenous oxide. 1 1

2. A composition adapted to inhibit acid attack on ferrous metals consisting essentially of the solution obtained by dissolving 40 parts by weight of arsenous oxide, 8 to 12 parts by weight of sodium hydroxide, and 8 to 12 parts by weight of boric acid in 60 to 350 parts by weight of water, said solution being characterized by the capacity to dissolve substantial amounts of carbon dioxide without precipitation of arsenous oxide.

3. A method of inhibiting the corrosion of ferrous metal surfaces in an oil well producing a well fluid comprising oil, brine and carbon dioxide gas, which comprises introducing into the well bottom an aqueous inhibitor solution characterized by the capacity to dissolve substantial amounts of carbon dioxide without precipitation of arsenous oxide and formed by dissolving 40 parts by weight of arsenous oxide, 8 to 12 parts by weight of sodium hydroxide, and 8 to 12 parts by weight of boric acid in 60 to 350 parts by weight of water, said solution being introduced in amount sufiicient to give the water produced by said well an inhibitor concentration in the range of 1-100 ppm. calculated as arsenous oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,105,839 McNutt Jan. 18, 1938 2,119,506 Watts May 31, 1938 2,635,698 Rohrback et a1 Apr. 21, 1953 2,658,036 Core et al Nov. 3, 1953 2,684,333 Rohrback et al July 20, 1954 2,805,915 Rohrback Sept. 10, 1957 

1. AS AN INHIBITOR FOR PREVENTING CORROSION OF FERROUS METAL SURFACES IN OIL WELLS, A SOLUTION CONSISTING OF THE FOLLOWING INGREDIENTS: 40 PARTS BY WEIGHT OF ARSENOUS OXIDE, 8 TO 12 PARTS BY WEIGHT OF SODIUM HYDROXIDE 8 TO 12 PARTS BY WEIGHT OF BORIC ACID, AND 60 TO 350 PARTS BY WEIGHT OF WATER 